Understanding Population Size and Ecosystems: WJEC A Level Biology Guide

Population Growth and Ecosystem Basics

Think about bacteria multiplying in a petri dish - that's population growth in action! A population is simply all the organisms of the same species living in a particular area, whilst an ecosystem includes both living things and their physical environment.

Population growth follows predictable patterns. During the lag phase, numbers barely change as organisms adapt to their new environment. Then comes the exponential phase where birth rates exceed death rates and population explodes - this only happens when resources are unlimited (which is rare in nature!).

Eventually, populations hit their carrying capacity - the maximum number the environment can support long-term. At this point, the stationary phase kicks in where birth and death rates balance out. If resources run out completely, populations enter the death phase.

Key insight: True exponential growth only occurs in laboratory conditions - in nature, something always limits population size eventually.

What limits population growth? Density-dependent factors like disease and food shortages get worse as populations increase. Density-independent factors like floods affect populations regardless of their size.

Competition and Feeding Relationships

Competition shapes every ecosystem you'll encounter. Intraspecific competition occurs between members of the same species (think two male stags fighting), whilst interspecific competition happens between different species occupying similar niches - their role in the ecosystem.

Here's a crucial rule: two species cannot occupy exactly the same niche in the same habitat. One will always outcompete the other! Animals compete for food, shelter and mates, whilst plants battle for light, space, water and nutrients.

Different organisms have evolved various feeding strategies. Autotrophic nutrition means making your own food (like plants), whilst heterotrophic nutrition involves eating others. Holozoic organisms have digestive systems, detritivores break down dead matter, and saprobionts secrete enzymes externally to digest their food.

Remember: Population fluctuations are normal and healthy - they're regulated by the balance between birth and death rates responding to environmental changes.

Parasitic nutrition involves one organism (the parasite) depending entirely on another (the host) for survival, usually harming the host in the process.

Energy Flow and Ecological Pyramids

Energy powers every ecosystem, but here's the catch - most of it gets lost at each trophic level! Only about 10% of energy transfers from one level to the next, which is why food chains rarely exceed five levels.

Primary productivity measures how fast producers create organic matter. Gross primary productivity (GPP) is the total energy captured, but plants use some for respiration, leaving net primary productivity (NPP) as the energy available to other organisms. Remember: NPP = GPP - Respiration.

Energy disappears through respiration (converted to heat), parts that aren't eaten (bones, fur), and waste that can't be digested. This explains why there are millions of grass plants but only a few tigers in any ecosystem.

Ecological pyramids help visualise these relationships. Pyramids of numbers show organism quantities, pyramids of biomass show total mass at each level, and pyramids of energy show energy content - these always have the same pyramid shape.

Fascinating fact: Of all sunlight hitting a leaf, only about 40% reaches the chloroplasts, and much of that is the wrong wavelength for photosynthesis!

Secondary productivity measures how efficiently consumers convert food into their own biomass.

Succession and Habitat Development

Imagine a bare rock slowly transforming into a thriving forest - that's succession in action! This predictable process occurs wherever new habitats form, from volcanic islands to abandoned car parks.

Primary succession starts from scratch with no soil present. Pioneer species like lichens are the brave first colonisers, tolerating harsh conditions that would kill most organisms. As they die, they create humus (partially decomposed organic matter) that forms the basis of soil.

Each seral stage creates better conditions for the next group of species. Mosses and ferns follow lichens, then grassland, scrubland, and finally woodland. The climax community represents the stable end point where species composition stops changing dramatically.

Secondary succession occurs where soil already exists (after fires, tree felling, or farming abandonment) and progresses much faster since the foundation is already there.

Cool example: On sand dunes, marram grass acts as a pioneer species with deep roots that bind shifting sand together, creating stable conditions for other plants to establish.

Early stages feature fast-growing, short-lived species that invest heavily in reproduction. Climax stages have longer-lived species with complex relationships and efficient nutrient cycling.

Carbon and Nitrogen Cycles

The carbon cycle and nitrogen cycle are nature's recycling systems, constantly moving essential elements between living and non-living components of ecosystems.

Carbon moves through photosynthesis (removing CO₂ from atmosphere), respiration (releasing CO₂), and decomposition. However, human activities have disrupted this balance. Burning fossil fuels contributes 70% of excess atmospheric CO₂, whilst deforestation accounts for 30% - we've removed half the world's forests in just 30 years!

The greenhouse effect occurs when solar radiation warms Earth's surface, but greenhouse gases trap the heat that would normally escape back to space. Your carbon footprint measures the greenhouse gases your activities produce through electricity, heating, and transport.

Nitrogen cycling is more complex because atmospheric nitrogen gas is largely unusable to most organisms. Nitrogen fixation by bacteria like Rhizobium (living in legume roots) converts nitrogen gas into ammonium ions that plants can absorb.

Essential process: Ammonification breaks down dead organisms, releasing ammonium ions that nitrifying bacteria convert to nitrites then nitrates - the forms plants prefer.

Denitrification returns nitrogen to the atmosphere, especially in waterlogged soils where anaerobic bacteria convert nitrates back to nitrogen gas.